Concentrated sulfuric acid (H2SO4) is widely used as a catalyst in industrial processes, such as olefin and isoparaffin alkylation. However, there are serious drawbacks to the use of H2SO4 in industrial process plants. Concentrated H2SO4 is extremely hazardous if inhaled, very hazardous in the case of skin or eye contact, and is considered a proven human carcinogen by IARC and OSHA.
In alkylation processes, H2SO4 is continuously diluted by the production of acid soluble oils, necessitating withdrawal and reprocessing of large volumes of the spent acid to restore catalytic activity. This reprocessing is typically performed by combusting and re-concentrating the acid in a separate processing plant. In many cases, the spent acid must be transported to the processing plant by tank trucks or rail cars, thereby creating a potential public exposure hazard due to road or rail accidents.
Efforts to develop safer, alternative catalysts in lieu of H2SO4 have encountered serious challenges, however. Hydrofluoric acid (HF) may be used as an alternative, although HF presents even greater hazards to humans than H2SO4. Further, solid alkylation catalysts have proved difficult to commercialize due to rapid fouling and deactivation.
FIG. 1A is a block diagram schematically representing a conventional H2SO4 alkylation unit 10, according to the prior art. H2SO4 alkylation unit 10 may include a hydrocarbon feed line 13, an H2SO4 alkylation reactor 14, and an acid settler 16 coupled to H2SO4 reactor 14 for separating an acid/hydrocarbon mixture into a hydrocarbon phase and an acid phase. The acid phase may be recycled to H2SO4 reactor 14. Alkylation unit 10 may further include an acid wash vessel 18, an alkaline water wash vessel 20, a fractionation unit 22, a product treatment unit 24, and an external refrigeration unit 26. External refrigeration unit 26 uses a conventional refrigerant, such as Freon or propane, for cooling reactor 14. Fractionation unit 22, which contains multiple distillation or fractionation columns, fractionates the hydrocarbon phase to provide one or more products for treatment by product treatment unit 24. In some instances, a conventional alkylation unit may include a feed treatment unit 12 for treating the hydrocarbon feed(s) to reactor 14.
FIG. 1B is a block diagram schematically representing a conventional H2SO4 alkylation unit 10′, also according to the prior art. Alkylation unit 10′ may include a hydrocarbon feed line 13, H2SO4 alkylation reactor 14, acid settler 16, acid wash vessel 18, alkaline water wash vessel 20, fractionation unit 22, and product treatment unit 24, substantially as described with reference to FIG. 1A. In contrast to unit 10 of FIG. 1A, alkylation unit 10′ includes an effluent refrigeration unit 26′ that uses effluent from the alkylation reactor as the refrigerant.
U.S. Pat. No. 5,284,990 to Peterson et al. discloses a method for converting an HF alkylation unit to a H2SO4 alkylation unit. The disclosure of the '990 patent is incorporated by reference herein in its entirety for all purposes.
The quest for an alternative catalytic system to replace conventional H2SO4 and HF catalysts in alkylation processes has been researched by various groups in both academic and industrial institutions. Thus far, no viable replacement catalyst for performing such processes has been commercialized.
Recently there has been considerable interest in metal halide ionic liquid catalysts as alternatives to H2SO4 and HF catalysts. As an example, the ionic liquid catalyzed alkylation of isoparaffins with olefins is disclosed in U.S. Pat. No. 7,432,408 to Timken, et al. Further, U.S. Pat. No. 7,572,943 to Elomari, et al. discloses the ionic liquid catalyzed oligomerization of olefins and the alkylation of the resulting oligomers(s) with isoparaffins to produce alkylated olefin oligomers.
PCT Publication No. WO 2011/015664 discloses a method for revamping an H2SO4 or HF alkylation unit to an ionic liquid alkylation unit, wherein one or more cyclone units are provided to promote the separation of ionic liquid from hydrocarbons.
Liu et al. (Oil & Gas Journal (2006) Vol. 104, Issue 40) describe retrofitting an H2SO4 alkylation unit for use in composite ionic liquid catalyzed alkylation by modifying the settler internals to enhance the separation of the composite ionic liquid from alkylate gasoline, and by providing a surge tank, wherein the composite ionic liquid is recycled from the surge tank to a STRATCO® (continuous stirred tank) reactor.
There is a need for the efficient and cost-effective conversion of existing, conventional H2SO4 alkylation units into ionic liquid alkylation systems suitable for ionic liquid catalyzed alkylation processes.